Components formed of specialty materials like superalloys are used in various industrial applications, under a diverse set of operating conditions. In many cases, the components are provided with coatings which impart several characteristics, such as corrosion resistance, heat resistance, oxidation resistance, and wear resistance.
Oxidation-resistant coatings are often critical if the underlying component is exposed to an oxidizing atmosphere for an extended period of time. This is especially true in the case of components formed of aluminum-containing superalloys, which are often used in gas turbine engines which operate at elevated temperatures, e.g., 1000° C.-1150° C. In the absence of a protective coating, the oxidizing atmosphere can deplete the superalloy of aluminum. Since aluminum can greatly enhance the oxidation-resistance of the protective coatings, the loss of aluminum can be detrimental to the integrity of the superalloy.
While increasing the aluminum content in the protective coatings often improves oxidation-resistance, the increase may be detrimental to other properties. For example, higher aluminum levels can decrease coating ductility, causing cracking in the coating during service. This in turn results in the loss of the hermetic, protective nature of the coating.
Many of the oxidation-resistant coatings for superalloys are formed from conventional alloys of the formula MCrAlX, where M is iron, nickel, or cobalt. “X” is one of the elements mentioned below, in a more detailed description of the coating. In many instances, the oxidation-resistant coating is used as the most external layer of a component, e.g., a turbine engine blade. In that case, the coating often has to be very smooth, for maximum aerodynamic efficiency.
Some of the thermal spray techniques are often used to deposit oxidation-resistant coatings with a desired surface texture. Examples of the thermal spray processes are high velocity oxy-fuel (HVOF) and vacuum plasma spray (VPS). Each of these techniques has attributes which make it suitable for a given situation. For example, VPS applications are often desirable when it is critical that the final coating be substantially free of internal oxides.
While these types of thermal spray techniques are quite suitable for applying oxidation-resistant coatings under many circumstances, they exhibit drawbacks in other situations. For example, VPS and HVOF techniques are sometimes not effective for applying the coatings to regions of a substrate which are somewhat inaccessible. The spray equipment may be too large and cumbersome for such regions. As an illustration, it can be very difficult to thermally spray a coating on a flange of a turbine engine part. Moreover, applying the coating to any internal cavity in the part can be problematic.
Furthermore, thermal spray processes may include one or more masking steps. These steps can be very time-consuming. Thus, it is often very difficult to carry out local repairs using the processes.
It should thus be apparent that new methods for efficiently applying oxidation-resistant coatings to a substrate would be welcome in the art. The methods should be capable of providing a coating with substantially the same quality as coatings applied by thermal spray processes. Moreover, it would be desirable if the new methods were capable of applying the coating to inaccessible regions of a substrate. Furthermore, the methods should be compatible with any other fabrication processes to which the substrate is being subjected. It would also be very beneficial if the new methods allowed one to readily change the composition of the oxidation-resistant coating, to satisfy the needs of a particular substrate. It would also be desirable if the coating method were applicable to local (small area) application, for “new make” and in-situ repair of oxidation-resistant coatings.